The `let` keyword is used to declare variables in JavaScript. The `var` keyword can also be used to declare variables, but the key difference between them lies in their scopes. `var` is **function scoped** while `let` is **block scoped** - we will discuss this in more detail later. 

To understand the `let` keyword, it is important to understand how it differs from `var`

# Function Scope
Consider the example below:

function func() {
  // x is known here but not defined.
  console.log('value of x here: ', x)
  {
    var x = 10;
    x = x + 5;
  }
  // x is still known here and has a value.
  console.log('value of x after for block: ', x)
}
// x is NOT known here.
func()

function func() {
  // x is NOT known here. Try uncommenting the line below.
  // console.log('value of x here: ', x)
  {
    let x = 10;
    x = x + 5;
  }
  // x is NOT known here. Try uncommenting the line below.
  // console.log('value of x after for block: ', x)
}
// x is NOT known here.
func()

In the code snippet above, the first tab uses `var`, note how `var` is declared in **line 5**, inside a block. This variable is known (but **undefined**) above the block. However, it is known and **defined** after the block. 

The highlighted lines represent the areas where `x` has a scope. Since it is the entire function itself, `var` is said to have **function** scope.

In the second code tab that declares `x` using `let` in **line 5**, note that it is only known and defined within the block it is declared in - the highlighted lines. It is out of scope everywhere outside its block.

> Try uncommenting all the `console.log(x)` statements in the code that uses `let` and note how JavaScript throws an error.

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# Block scope

Consider the example below which will highlight the block scope for `let` more clearly:

let mango = "yellow"

if (mango === "yellow"){
  let mango = "blue"
  console.log(mango)
}
console.log(mango)

var mango = "yellow"

if (mango === "yellow"){
  var mango = "blue"
  console.log(mango)
}
console.log(mango)

## Using `let`
`mango` is declared as ***"yellow"*** in **line 1**, and redeclared as ***"blue"*** in **line 4** - inside the `if` block. However, as the output shows, the `mango` declared inside the `if` block has scope within that block only. Outside this block, the original `mango` variable set to ***"yellow"*** is available.

## Using `var`
Conversely, in the code that uses `var`, redeclaration of `mango` to ***"blue"*** inside the `if` block, also changes the `mango` declared in **line 1** to ***"blue"***. This is because variables declared with `var` are defined globally, regardless of block scope. 

What is the let keyword in JavaScript?

Function Scope Consider the example below: 

What is the let keyword in JavaScript?

Function Scope

Block scope

Using let

Using var

Using `let`

Using `var`